1. Field of the Invention
The present invention relates to an idling speed control device of an engine.
2. Description of the Related Art
In a fuel injection type engine, the basic amount of fuel injected by a fuel injector is usually calculated from the engine speed and the level of vacuum in the intake passage, or from the engine speed and the amount of air fed into the engine cylinder, and the actual amount of fuel injected by the fuel injector is feedback-controlled so that the air-fuel ratio of mixture fed into the engine cylinder becomes equal to a predetermined desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, by correcting the basic amount of fuel on the basis of the output signal of the oxygen concentration detector (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine. Nevertheless, even if such a feedback control is carried out, when the amount of fuel injected by the fuel injector is abruptly increased as at the time of acceleration, the amount of fuel adhering to the inner wall of the intake port in the form of a liquid fuel is increased. Since this liquid fuel is not fed into the engine cylinder immediately after adhering to the inner wall of the intake port, the air-fuel mixture fed into the engine cylinder temporarily becomes lean. Conversely, when the engine is decelerated, the absolute pressure in the intake port becomes low. As a result, since the amount of vaporization of the liquid fuel adhering to the inner wall of the intake port is increased, the air-fuel mixture fed into the engine cylinder temporarily becomes rich.
Consequently, in a fuel injection type engine, the amount of fuel injected by the fuel injector is usually increased at the time of an acceleration and decreased at the time of a deceleration, so that the air-fuel ratio of the mixture fed into the engine cylinder becomes equal to a desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, even if the engine is operating in a transition state such as an acceleration state and a deceleration state. Consequently, in such a fuel injection type engine, the air-fuel ratio of mixture fed into the engine cylinder is controlled so that it becomes approximately equal to the desired air-fuel ratio, regardless of the operating state of the engine.
Nevertheless, in such a fuel injection type engine, blowby gas and lubricating oil, for example, pass through the clearance between the valve stem and the stem guide of the intake valve and flow into the intake port, and thus, when the engine is run for a long time, carbon particles, etc., contained in the blowby gas and the lubricating oil are gradually deposited on the inner wall of the intake port and the rear face of the valve head of the intake valve. These deposited carbon particles, i.e., the carbon deposit, have a physical characteristic of retaining liquid fuel, and thus, if the carbon deposit is deposited on the inner wall of the intake port etc., the amount of liquid fuel adhering to the inner wall of the intake port, etc., is increased, and this increases the time taken by the liquid fuel to flow into the engine cylinder, after the liquid fuel adheres to the inner wall of the intake port, etc. Consequently, although the air-fuel ratio of mixture fed into the engine cylinder can be controlled so that it becomes approximately equal to the stoichiometric air-fuel ratio, regardless of the engine operating state, while the engine is relatively new, if the deposit is deposited on the inner wall of the intake port, etc., after the engine has been run for a long time, since the time taken by the liquid fuel to flow into the engine cylinder is increased, as mentioned above, the air-fuel mixture fed into the engine cylinder becomes lean at the time of acceleration. In addition, since the amount of the liquid fuel adhering to the inner wall of the intake port etc. is increased, the air-fuel mixture fed into the engine cylinder becomes rich at the time of deceleration. At this time, since the amount of the deposit is increased, the air-fuel mixture becomes even leaner at the time of acceleration and even richer at the time of deceleration. In this case, for example, the leaner the air-fuel mixture at the time of acceleration, the longer the time for which the air-fuel mixture remains lean.
Consequently, in a known fuel injection type engine, the time during which the air-fuel mixture becomes lean (hereinafter referred to as a lean time) within a fixed time after the accelerating operation of the engine is started and the time during which the air-fuel mixture becomes rich (hereinafter referred to as a rich time) within the fixed time after the accelerating operation is started, are calculated, and the acceleration increase in the amount of fuel fed by the fuel injector is corrected on the basis of the lean time and the rich time, so that the air-fuel ratio of mixture fed into the engine cylinder becomes a desired air-fuel ratio even if the accelerating operation of the engine is carried out (see U.S. Pat. No. 4,499,882).
As mentioned above, when the carbon deposit adheres to the inner wall of the intake port, etc., the air-fuel mixture becomes lean at the time of acceleration. But if the deposit adheres to the inner wall of the intake port, etc., the air-fuel mixture also becomes lean when the engine is started. That is, at the time of starting the engine, when the injection of fuel is started, since a large amount of fuel thus injected is retained by the deposit on the inner wall of the intake port, etc., the air-fuel mixture becomes overlean when and immediately after the engine is started. As a result, it is difficult to start the engine, or even if the engine can be started, the idling operation of the engine will not be stable thereafter.
Therefore, in a known engine, to ensure an easy start of the engine, when the engine is started, the amount of fuel fed into the engine cylinder is increased as the amount of the deposit is increased (see Japanese Unexamined Patent Publication No. 61-129435).
Two methods of preventing the air-fuel mixture from becoming very lean due to the presence of the deposit when the engine is started, can be considered: (1) to increase the amount of fuel injected by the fuel injector, or (2) to increase the amount of air fed into the engine cylinder. When comparing these methods, however, the increase in the amount of air has a much greater influence on preventing the air-fuel mixture from becoming lean. Namely, even if the amount of fuel is increased when the engine is started, as described in the above-mentioned Japanese Unexamined Patent Publication No. 61-129435, since the fuel injected by the fuel injector is retained by the deposit, it is impossible to prevent the air-fuel mixture from becoming overrich immediately after the engine is started. Consequently, an increase in the amount of fuel at the time of starting of the engine will have no practical effect.
Conversely, when the engine is started, if the velocity of air flowing within the intake port is increased, since the vaporization of fuel injected by the fuel injector is promoted, the amount of fuel adhering to the deposit is reduced. Further, if the velocity of the air is increased, the vaporization of fuel adhering to the deposit is promoted, and the fuel adhering to the deposit is sucked by the air and fed into the engine cylinder at an earlier stage after the fuel has adhered to the deposit. As a result, it is possible to prevent the air-fuel mixture from becoming overlean. In this case, if the amount of fuel is increased, although the amount of fuel fed into the engine cylinder is increased, this increase in the amount of fuel fed into the engine cylinder is effected by increasing the amount of the fuel. Consequently, from a lean prevention point of view, the increase in the amount of fuel at the time of starting the engine has only a secondary effect